The FCC has made a swath of 1200 MHz available for Wi-Fi, which must respect the rights of services already using the frequency band. Here’s why and how.

On April 23 2020, the Federal Communications Commission (FCC) voted unanimously to adopt sweeping new rules for wireless operation in the 6 GHz band [1], opening 1200 MHz of spectrum to unlicensed devices. Specifically, the FCC approved the rules it informally released to the public about three weeks prior to this momentous vote [2].

This development has been widely hailed as a big win for Wi-Fi—and rightly so—but what exactly did the FCC decide and what does the new regulation mean for Wi-Fi and the 6 GHz band? In this article, we provide an overview of the 6 GHz band, the process that lead to the new regulation and the details of the rules that the FCC has approved for operation of unlicensed devices in 6 GHz.

The 6 GHz band incumbents
The spectrum that is commonly referred to as “the 6 GHz band” consists of the frequencies between 5.925 GHz and 7.125 GHz, totaling 1200 MHz of spectrum. Prior to the April 23 announcement from the FCC, the 6 GHz band was designated as exclusive non-federal spectrum and licensed to several radio services. Figure 1 illustrates the use of the 6 GHz bands by the incumbent services.

Fixed Service (FS) and Fixed-Satellite Service (FSS) operations are spread throughout most of the 6 GHz band. Fixed Service is used for reliable point-to-point microwave links. These links support a variety of services such as public safety, management of electric grids, long-distance telephone service and backhaul. FSS is allowed for Earth-to-space communication in all but the upper 150 MHz of the 6 GHz band. FSS uses these frequencies for content distribution, live news and sports events, cable television and backhaul of telephone and data traffic.

The FCC allows mobile operations in the 6.425 GHz to 6.525 GHz band and 6.875 GHz to 7.125 GHz band. These services include Broadcast Auxiliary Service (BAS), Cable Television Relay Service (CARS) and Local Television Transmission Services (LTTS).
Figure 2 shows the number of licensed user assignments in the 6 GHz band, normalized per megahertz.

Protection of these existing incumbent users was a challenge in allowing additional non-licensed operation in the 6 GHz band. As we will see later, some of the new regulatory rules explicitly address this issue.

In addition to the licensed fixed and mobile services, ultra-wideband (UWB) systems may operate in the 6 GHz band on a non-interference basis. It’s important to note, however, that UWB is not considered a licensed user of the band.

Changing regulation in 6 GHz
Noting the “explosive demand” for unlicensed spectrum, primarily driven by Wi-Fi, the FCC embarked on a process to investigate the possible use of the 6 GHz band for unlicensed services.

The FCC regulates interstate and international communications by radio, television, wire, and satellite across the US. It is also responsible for implementing and enforcing communications law and regulations. When considering changes to regulations, the FCC employs a process known as “notice and comment” rulemaking. Under this process, the FCC first provides notice to the public about its intention to modify existing rules. Each of these notices is followed by a period of public comments where interested parties can submit feedback on the tentative proposals. This feedback is considered by the FCC before issuing its final rules on the subject.

As part of the process the FCC can publish a number of different documents:

• Notice of Inquiry (NOI): this is an initial invitation to the public to allow the FCC to gather information on a broad subject. An NOI about the use of 6 GHz by unlicensed users was issued by the FCC in July 2017 [3];
• Notice of Proposed Rulemaking (NPRM): the NPRM contains more detail on the proposed changes to the Commission’s rules. It may follow an NOI. Each NPRM is followed by a period of at least 30 days where comments can be submitted to the FCC. The FCC issued its NPRM on unlicensed use of the 6 GHz Band in October 2018 [4];
• Further Notice of Proposed Rulemaking (FNPRM): An FNPRM may be issued after review of comments on NPRM and provides an opportunity to comment further on a related or specific proposal;
• Report and Order (R&O): this is issued after considering comments to a NPRM or FNPRM and specifies new rules, amends existing rules or makes a decision not to do so. After the approval of the R&O, the new or amended rules become effective 30 days after publication in the Federal Register.

The 6 GHz NPRM [4] generated significant interest from both proponents of unlicensed operations and representatives of incumbent services. Some 115 companies or organizations commented on the NPRM. During the subsequent discussion period, over 100 technical studies were submitted to the FCC to address specific issues, simulate performance impacts, investigate technical claims, etc.

The FCC duly considered the provided input and issued its R&O in April 2020. At the same time, it also published an FNPRM to solicit input on additional topics that were not part of the original NPRM.

New rules for unlicensed operation in 6 GHz
Under the FCC nomenclature for unlicensed use, the 6 GHz spectrum is now further subdivided into four Unlicensed National Information Infrastructure (U-NII) radio bands: U-NII-5 to U-NII-8. Some of the rules adopted by the FCC depend on which U-NII band the unlicensed device operates. The edges of the U-NII bands coincide with the frequency ranges that currently divide the operating frequencies of the licensed 6 GHz services (Fig.1).

Figure 3 shows the frequencies and edges of the various parts of the 6 GHz band.

In its updated regulation, the FCC now allows two types of unlicensed devices to operate in 6 GHz:

• Low-power Access Points (APs) and their associated client devices;
• Standard-power Access Points (APs) and their associated client device.

Each class of devices comes with its own set of regulatory rules.

Low Power Operation
Low-power APs may operate in the 6 GHz band, provided they comply with the following transmit power requirements:

• Total transmitted power below 30 dBm EIRP;
• Power Spectral Density limited to 5 dBm/MHz EIRP or less;
• Meet the emission mask and an emission limit of -27 dBm/MHz limit at frequencies below the bottom of the U-NII-5 band and above the upper edge of the U-NII-8 band.

[EIRP is the amount of power allowed for a radiator that emits energy uniformly (“isotropically”) in all directions. Directional antennas adjust their effective transmit power such that the energy radiated in the direction of the strongest beam does not exceed that of an isotropic antenna along the same direction.]

This corresponds to, for instance, 18 dBm in 20 MHz, 21 dBm in 40 MHz, 24 dBm in 80 MHz, 27 dBm in 160 MHz and 30 dBm in 320 MHz. Bandwidths higher than 320 MHz are not allowed.

Low-power APs are confined to indoor use only and are allowed to use the complete 6 GHz spectrum, i.e. U-NII-5, U-NII-6, U-NII-7 and U-NII-8. This amounts to 1200 MHz of new spectrum.
The FCC felt that the combination of lower power and mandatory indoor use were sufficient to afford the required protection to incumbent services in the 6 GHz band. Since indoor use (and its resulting signal attenuation towards outside areas) is a crucial assumption in the interference that will be generated by these devices, the FCC further imposes restrictions that are intended to prevent the use of low-power APs outdoor. Specifically:

• Low-power APs can’t be weather resistant, preventing them from being installed outside permanently;
• Low-power APs can only have integrated antennas and it is prohibited to provide the capability of connecting other antennas to the devices;
• Low-power APs can’t be battery operated;
• Low-power APs must be clearly labeled with a notice that states that “FCC regulations restrict operation of this device to indoor use only.”

While none of this is full proof, the intention is clearly to only use these devices indoors.
Low-power APs are not allowed to operate while in motion, with exception of large passenger aircraft operating over 10,000 feet.
Client devices that are associated with the low-power AP must operate at the following power levels:

• Total transmitted power below 24 dBm EIRP;
• Power Spectral Density limited to -1 dBm/MHz EIRP or less.

There are exceptions such as client devices operating in a mesh networks or as Wi-Fi extenders. In those cases, the power limits of the low-power AP apply, provided that the client devices also follow the relevant AP rules and are all located within a single structure.

Standard Power Operation
Standard-power APs are allowed to operate at the following transmit powers:

• Total transmitted power below 36 dBm effective isotropic radiated power (EIRP);
• Power Spectral Density limited to 23 dBm/MHz EIRP or less;
• Meet the emission mask and an emission limit of -27 dBm/MHz limit at frequencies below the bottom of the U-NII-5 band and above the upper edge of the U-NII-8 band.

In this case, the power-spectral density (PSD) requirement allows the device to always operate at 36 dBm EIRP for all bandwidths. As for low-power access points (APs), bandwidths higher than 320 MHz are not allowed.

The significantly higher power permitted for standard-power APs comes at a price. These devices are only allowed in U-NII-5 and U-NII-7, accounting for about 850 MHz of spectrum. Importantly, even in these bands, standard-power APs can operate only under the direction of a so-called Automated Frequency Coordination (AFC) system. An AFC system is aware of the location of incumbent services and will inform the AP of which parts of the spectrum it may operate, based on the APs location, antenna height, and transmit power. The R&O provides extensive details on the operation of the AFC system, which we’ll discuss below.

The AP must determine its geographical location to provide it to the AFC. This geolocation capability can be either internal (like a built-in GPS) or external (provided by a connected device). The AP also needs to provide its antenna height above ground to the AFC system. This information can be configured manually or be determined automatically.

The AP is expected to contact the AFC system at least once a day to check for changes to the allowed frequencies. If the two can’t establish a connection, the AP is required to stop operations after a 24-hour grace period.

AFC is intended mainly to protect Fixed Services, but standard-power APs are also expected to limit their upward radiated power (above 30°) to 21 dBm to protect Fixed Satellite Services.

Like low-power APs, Standard-power APs must not operate while in motion, with exception of large passenger aircraft operating over 10,000 ft.

Client devices that are associated with the standard-power AP must operate at the following power levels:

• Total transmitted power below 30 dBm EIRP
• Power Spectral Density limited to 17 dBm/MHz EIRP or less

The final power the AP is, however, allowed to operate at may be reduced by the AFC for specific frequencies. If the final allowed power for the AP is below 36 dBm, the allowed power for its associated client devices needs to be reduced by the same amount. Specifically, the allowed maximum power for client devices associated with standard-power APs is always 6 dB below the allowed maximum power for the AP – whatever that level may be.
Table 1 summarizes the power requirements for both types of AP and their associated clients.

Automated frequency coordination
A major new requirement for Wi-Fi systems (or for that matter any unlicensed system wanting to operate in the 6 GHz band using standard Tx power) is the requirement to operate under the direction of an AFC system. The published R&O lays out requirements for the AFC system as well.

The AFC’s primary function is to protect fixed-service microwave links. In a secondary role, it protects frequencies used by certain radio astronomy observatories. The AFC’s knowledge of microwave links is based on the Universal Licensing System (ULS) database, which is an existing official database for licensed microwave links in the U-NII-5 and U-NII-7 bands. The AFC system is supposed to synchronize with the ULS database daily.

After being contacted by an AP device, the AFC system will determine suitable channels for this AP based on a model of the interference that this AP would generate. The calculation (Figure 4) will establish an “exclusion zone” where the AP would generate interference towards incumbents that exceed allowable limits. This calculation needs information about the AP’s location and antenna height. To model the interference, the AFC system assumes a channel model to simulate the interference generated by the AP. The R&O explicitly specifies the channel model and interference criterion that should be used for this calculation.

The final feedback from the AFC system to the AP is a list of channels that the AP may use at a range of different powers.
The AP identifies itself to the AFC system using a unique identifier that consists of the device’s FCC identifier (FCC ID), and its serial number. The AFC system can use this to authenticate the device. The identifier can also be used for interference mitigation and enforcement purposes.

The R&O doesn’t provide details on the communication protocol between the AP and the AFC system, but it does require that any such communication, as well as the data being stored by the AFC system, should meet advanced security standards.

AFC systems will be run by “AFC operators”. An AFC operator will need an official designation, which will be administered by the Office of Engineering and Technology (OET). The OET will “license” AFC operators for a five-year term that the FCC can renew. To provide the AFC service to standard-power APs, AFC operators will be allowed to charge a fee. To fully address all aspects of AFC operation, the FCC encourages the industry to convene a group of interested stakeholders to address issues such as interference detection and mitigation, security measures, testing and certification procedures, etc.

FNPRM
In addition to the R&O that spells out the new requirements for unlicensed operation in the 6 GHz band, the FCC also issued a new FNPRM to solicit input on four new questions related to operation in 6 GHz:

• Very Low Power Operation;
• Power Spectral Density Increase for Low Power Indoor Operation;
• Mobile Standard-Power Access Point Operation;
• Higher Power Limits and Antenna Directivity for Standard-Power Access Points.

First, the FCC seeks input on the operation of so-called “very low-power devices.” The tentative proposal is for these devices to operate at 14 dBm EIRP total power and -8 dBm/MHz PSD EIRP. The intention is to allow these devices in the entire 6 GHz band (U-NII-5 to U-NII-8) for both indoor and outdoor use. Given the lower power, it is suggested that these devices should not be required to use AFC, even for outdoor operation. The FNRPM proposes to allow this very-low power mode but is looking for input on the exact power levels and ways to mitigate interference to incumbent users.

Secondly, the FNPRM invites additional comments on the PSD limit of low-power APs. That level is currently specified at 5 dBm/MHz PSD EIRP. The FCC is asking for input on whether this can be increased to 8 dBm/MHz PSD EIRP, with a maximum permissible EIRP of 33 dBm when a device uses a bandwidth of 320 MHz.

The third item pertains to standard-power access points. These APs are currently allowed in fixed deployments only. The FNPRM seeks input on whether to allow standard-power access points, under AFC control, to be used in mobile applications. Specifically, the impact on AFC needs to be considered.

The fourth item addresses a specific use case for standard-power APs. While the current allowed power level for these devices is set at 36 dBm EIRP, the FNPRM is asking whether this level can be increased for devices that serve point-to-point links with highly directional antennas. As a related question, the FNRPM also seeks input on whether antenna directionality could be incorporated in the AFC system framework.

The comments and discussion generated by the responses to the FNPRM may results in further changes or additions to the rule for unlicensed operation in the 6 GHz band.

Conclusion
With its decision to open the 6 GHz band to unlicensed operation, the FCC has provided up to 1200 MHz of extra spectrum for Wi-Fi operation. This decision will certainly redefine Wi-Fi as we know it, but it’s important to understand that the operating rules in 6 GHz are somewhat different from what Wi-Fi is used to in the 2.4 GHz and 5 GHz bands.

References
[1] “FCC Adopts New Rules for the 6 GHz Band, Unleashing 1,200 Megahertz of Spectrum for Unlicensed Use”, FCC news release, https://docs.fcc.gov/public/attachments/DOC-363945A1.pdf
[2] 6 GHz Unlicensed R&O/FNPRM, https://docs.fcc.gov/public/attachments/FCC-20-51A1.pdf
[3] Exploring Flexible Use in Mid-Band Spectrum Between 3.7 GHz and 24 GHz, Notice of Inquiry, GN Docket No. 17-183, http://transition.fcc.gov/Daily_Releases/Daily_Business/2017/db0713/DOC-345789A1.pdf
[4] Notice of Proposed Rulemaking, FCC 18-147, https://ecfsapi.fcc.gov/file/1024814219781/FCC-18-147A1.pdf.

Sigurd Schelstraete is currently senior technical director at ON Semiconductor. Before that, he was senior architect at Quantenna Communications, which was acquired by ON Semiconductor in June 2019. In these roles, he has been responsible for system design and algorithm development for Quantenna’s Wi-Fi chips. He’s also the lead technical representative for ON in various standardization bodies such as IEEE 802.11 and Wi-Fi Alliance, where he is involved in the definition of next-generation wireless technologies.

Related articles

• Wi-Fi 6E cometh: FCC opens 1200 MHz of spectrum to unlicensed communications
• Wi-Fi 6E is coming, chipsets with MIMO now sampling
• What Every Engineer Should Know About 5G/Wi-Fi Handoff Security
• WLAN 2×2 MIMO Signaling Tester for IEEE 802.11ax
• 802.11ax: Are we there yet?
• Solving Test and Deployment Challenges for 802.11ax
• Test 802.11ax signal quality for transmitters and receivers